Method of binding a phenol-based thermoplastic resin to a cured and molded-thermoset phenolic plastic

ABSTRACT

A method of making grinding wheel mounts by molding a central hub onto a thermoset plastic abrasive disc is provided. An aromatic polycarbonate thermoplastic resin is molded to form a hub on a cured thermoset plastic abrasive disc. It is believed that the polycarbonate forms both a mechanical and chemical bond with the thermoset plastic to provide an efficient method of producing grinding wheels.

This is a division of application Ser. No. 632,026 filed Nov. 14, 1975,now U.S. Pat. No. 4,088,729, in turn a continuation of Ser. No. 450,261,filed Mar. 11, 1974, abandoned, and in turn a divisional of Ser. No.108,789 filed Jan. 22, 1971 now U.S. Pat. No. 3,800,483.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in the manufacturing of abrasivegrinding wheels.

2. Prior Art

The concept of utilizing a revolving surface for frictional or impactremoving of rough spots on various items such as weldments, concretesurfaces, machine work, and the like is well known. The U.S. Pat. No.3,081,584 granted Mar. 19, 1963 to Robert A. Bullard, discloses aconventional abrasive wheel presently in commercial use. This abrasivewheel consists basically of a central base member to provide connectionto a portable hand grinder or other machine for supplying the motiveforce and a disc member attached to the central base member. The discmember has usually assumed the form of a screen impregnated with anabrasive and a bonding agent.

The abrasive wheel of the Bullard patent utilizes a central mountingmember with a considerable flange for providing sufficient surface topermit a fastening of the abrasive disc onto the central member by meansof an epoxy resin cement. To further insure a sufficient bonding betweenthe central mounting member and the abrasive disc, the tubular portionof the central mounting member that extends through the abrasive disc isgenerally "ball swaged" to provide a counter sunk portion of the tubularprojection against the lower surface of the abrasive disc. Thus, themanufacturing process of the presently accepted abrasive wheel requiresa two-step process for joining an abrasive disc to a metallic centralmounting member. This process includes the step of adhering by an epoxyresin the flange of the central mounting member to the disc and furtherthe step of mechanically deforming the metallic central member bycountersinking or "ball swaging" the central mounting member onto thelower surface of the abrasive disc. Generally the epoxy material must becured before the abrasive wheel is capable of use.

Another form of an abrasive wheel used for "snagging" provides areplaceable disc having a central hole which is mounted by a coaction ofwashers or clamps on either side of the disc onto a grinder machine.Vibrational problems in loosening the washers and clamps have been aproblem in utilizing this form of abrasive wheel. Likewise aconsiderable amount of time and effort must be utilized by the operatorin mounting the abrasive wheel on the grinder or changing an abrasivedisc.

SUMMARY OF THE INVENTION

The present invention is designed to provide a method or process ofmanufacturing abrasive wheels in a highly economical and efficientmanner. This is accomplished by molding a strong thermoplastic mountinghub directly onto the abrasive wheel. By appropriately designing theabrasive wheel the thermoplastic hub can be molded to form both achemical and mechanical bond in a single step that removes any necessityof an elaborate finishing process which would include a prolonged curingprocess of an epoxy adhesive material. The present invention also isdirected to a method of bonding a phenol-based thermoplastic resin to acured and molded thermoset phenolic plastic as will be hereinafterdescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

The best mode contemplated of carrying forth the process of the presentinvention will be described in detail below in connection with theaccompanying drawings in which:

FIG. 1 is a perspective view of an abrasive wheel produced according tothe process of the present invention;

FIG. 2 is a perspective cross-sectional view of the abrasive wheel ofFIG. 1;

FIG. 3 is a sectional view of the molding equipment utilized in theinventive process; and

FIG. 4 is a cross-sectional view of the molding apparatus of FIG. 3during operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The process of the present invention is generally accomplished byinserting a specially designed abrasive wheel, which may be preheated,into the stationary mold half of one side of a conventional plasticmold, and inserting a polished chrome plated hardened steel insertthrough the abrasive wheel disc by closing the die to form the moldcavity. A molding machine forces a thermoplastic into the mold and morespecifically, in the preferred process the thermoplastic material is ofthat family known as polycarbonates sold under the trademark MERLON,which is a thermoplastic resin that is manufactured by the process ofeither, phosgenation of dihydric phenols, usually bisphenol-A; or esterexchange between diaryl carbonates and dihydric phenols, usually betweendiphenyl carbonate and bisphenol-A; or interfacial polycondensation,generally accomplished with a chlorinated aliphatic solvent, aqueoussodium hydroxide, bisphenol-A and phosgene. The present processgenerally requires the polycarbonate polymer produced by any one of theabove methods to be heated to a temperature range of approximately 400°F. to 600° F. so as to be in a softened state. The general basestructure of the polycarbonate polymer is as follows: ##STR1## whereX=50-400 units

U.S. Pat. Nos. 3,030,331 and 2,999,845 granted to E. P. Goldberg can bereferred to for greater detail on polycarbonate polymers.

This polycarbonate resin can be considered an ester of carbonic acid andbisphenol A and generally has the following physical characteristics:

    ______________________________________                                        Compression ratio    1.74-5.5                                                 Specific gravity (density)                                                                          1.2                                                     Specific volume, cu. in./lb.                                                                       23.0                                                     Softening Range °F.                                                                         400-600                                                  Tensile strength, p.s.i.                                                                           8,000-9,500                                              Elongation, %        100.0-130.0                                              Flexural yield strength, p.s.i.                                                                    13,500                                                   Impact strength, ft. lb./in. of                                                                    12.0-17.5                                                notch (1/2 × 1/2 in. notched bar),                                      Izod test            (1/2 × 1/8                                                              in. bar)                                                 Compressive strength, p.s.1.                                                                       12,500                                                   ______________________________________                                    

Some possible injection molding parameters, for the aromaticpolycarbonate are as follows:

1. Cylinder, or stock temperatures of the material,--400° F. to 600° F.Most parts can be molded in the range of 500° F. to 580° F., but as partconfiguration, and shot size compared to the capacity of the machinewill vary, it is sometimes necessary to use the lower or the highertemperatures.

2. Mold Temperature--150° F. to 250° F., some molds do not have to beheated or cooled, but the average ideal mold temperature for mostpolycarbonate parts is, 180° F.

3. Injection Pressure--5,000 p.s.i. to 30,000 p.s.i. The average isusually from three quarters, to the full amount that the machine cansupply, 20,000 p.s.i. at the orifice of the nozzle gives the bestpolycarbonate parts, as a rule.

4. Cycle. This will depend on the part to be molded, however; parts aregoverned by their thickness, and it is necessary to inject the materialrapidly for most parts, but for very thick parts, slower injection isrequired, thus, slowing the overall cycle. Parts of good quality, 1/4"thick, have been molded in twenty seconds. Because of its high heatdistortion point, polycarbonate can be molded, using shorter cycles thanmost other plastics, as it cures in the mold, or sets up, quite rapidly.

Other thermoplastic resins that are capable of being used in place ofpolycarbonate are polypropylenes, both homopolymer and copolymer and theA.B.S. copolymer or terpolymer family of plastic (a combination ofacrylonitrile, butadiene and styrene). The high impact strength and highheat distortion characteristics make them suitable for the presentinvention.

During the process of one form of the invention a compatibility betweenthe particular thermoset plastic utilized in the abrasive disc of thisinvention has been found with the aromatic polycarbonate thermoplasticresin of the central hub member. In fact, it is believed that thethermoset plastic forms a chemical bond with the thermoplastic resin toincrease the strength of the connection between the hub and the abrasivedisc. Generally, the abrasive disc comprises a base of a fiberglass webor mesh containing abrasive powder of a well known type such as aluminumoxide and a phenolic resin to serve as a bonding agent.

Turning now to the thermoset plastic material applicable to the presentinvention, it has been found that phenol-formaldehyde resins prepared bythe one-stage or two-stage method are extremely suitable for use in thepresent invention.

In phenol-formaldehyde resins prepared by the "one-stage" method, theformaldehyde in a concentration greater than one mol per mol of phenolis used with an alkaline catalyst, such as caustic soda in order toproduce an ammonia-free resin.

In the "two-stage" method, an excessive amount of phenol together withformaldehyde and an acid catalyst, such as hydrochloric or sulfuricacid, are reacted to form what is known as a novalak. In this method,additional formaldehyde is required in order to complete the finalpolymerization, and this is usually attained in the form ofhexamethylenetetramine (commonly known as hexa).

Between the two types of phenol-formaldehyde resins, those produced bythe two-stage method are preferred for use in this invention, since theyare known to have a far longer shelf life than those prepared by theone-stage method.

Illustrative of these phenol-formaldehyde resins and specifically thenovalak resins, are the so-called Durez Resins produced by the HookerChemical Company.

These resins have a mean molecular weight of about 500 and an averagemolecular weight ranging from 450 to 550.

While not wishing to be limited to any particular theory, it is believedthat the resulting strength characteristics of the grinding wheel of thepresent invention are attained through the cross-bonding between thethermoset plastic and thermoplastic polycarbonate during the moldingprocess. In addition, the possibility of a Vander Wal bonding resultingfrom the interaction of the molecular poles of the polymers appears tobe a distinct possibility.

At this juncture, it should be mentioned that the phenolic resinscontemplated for use herein are not limited in number. That is, anyphenol-formaldehyde resin prepared in the aforementioned manner and fromthe aforementioned components will suffice, providing, however, it doesnot detract from the goals of the present invention.

However, the salient feature here resides in the fact that thethermoplastic component (polycarbonate) and the thermoset component(phenol-formaldehyde resin) when united, form an extremely strong bond.This is totally unexpected in view of the known properties between thesetwo types of materials. Hence, the thermoset plastic (herein defined asone which is already cured and being in the molded state) would not beexpected to create the strong chemical and mechanical bond as achievedherein.

As an additional feature of the invention it would be possible toprovide color coding in the plastics so that the color of the hub of aparticular abrasive wheel would signify a particular size or type ofdisc thus removing any necessity to provide other indicia that would,for example, state a specific safety limit for the use of the abrasivewheel.

Referring to FIG. 1, the end product of the manufacturing process can beseen. The grinding wheel assembly 10 comprises basically an abrasivedisc 12 and a hub 14.

The abrasive disc generally comprises a number of layers of fiberglassmesh impregnated with a mixture of an abrasive, such as aluminum oxideor silicon carbide or some other appropriate abrasive with a bondingagent which in the preferred embodiment is a resinoid organic bondingagent. Other bonding agents could include vitrified, rubber, shellac orsilicate. The hub member 14 in the preferred embodiment is an aromaticpolycarbonate molded into the shape of a hexagonical nut 18. The base ofthe hub 14 includes a flange 16 which is connected with the hexagonicalnut portion 18 by a generous radius 20 to remove any stress associatedwith a "notched effect."

As can be seen in FIG. 2, the hexagonical nut 18 has a threaded aperture22 and generally the top portion of the hexagonical nut 18 isappropriately countersunk. The lower surface of the abrasive disc 12 hasa circular portion 24 removed to form a cavity. This permits the heatsoftened aromatic polycarbonate to form a round ring of plastic 26 toprovide a mechanical bond between the hub 14 and the abrasive disc 12.Just as in the top portion of the hub 14 the lower portion is generallycountersunk and after the finishing operation, the hub 14 does notprotrude below the lower surface of the abrasive disc 12. Utilizing anaromatic polycarbonate having a tensile strength in the range of from8000 to 9500 psi, the preferred grinding wheel assembly will have a hubthat is bonded to the abrasive disc 12 and which will have a strengthstronger than the abrasive disc 12 itself. This form of polycarbonatematerial generally has an impact strength with values nearing that ofmost metals per unit weight.

Referring to FIGS. 3 and 4, the die assembly 28 comprising a movablemold half 30 and a stationary mold half 32 can be seen. On the back ofthe stationary mold half 32 is a locating ring 34 used to align the moldinto the molding machine. The nozzle 36 of the molding machine passesbetween the locating ring and seats against the sprue bushing 38. A backplate 40 supports the locating ring 34 and provides a slot 42 with thestationary mold half 32. The slot 42 accommodates the dogs or clampsthat are used to fasten the mold to the machine platen. Locating pins 44extend from the surface of the stationary mold half 32. An alignmentplate 46 is appropriately located with alignment dowels 48 to positionthe abrasive disc 12 in the die assembly. The movable mold half 30 hasrecesses which form female mates 50 for the locating pins 44 of thestationary mold half 32. A central cavity 52 permits the movable half ofthe mold 30 upon opening, to move back, allowing the ejector pin 51 tomake contact with the end of the insert 54, thus pushing the entiremolding free from the movable half of the mold. The insert 54 ispreferably made of hard machine steel, chrome plated for both wearresistance and for easy removal from the molded part, has a hexagonicalbase 56, for positioning in the movable mold half 30. The centralportion 58, of the insert 54, is round, and terminates in a threadedportion 60. A depression or cavity surrounds the insert 54 on the faceof the movable mold half 30. This undercut depression 64 serves twopurposes. It prevents the hot thermoplastic from flashing over thebottom surface of the abrasive disc 12 and it further serves as a spruepuller, that is, it has enough undercut to hold the molded assembly tothe face of the movable mold half until the ejector pin 51 contacts theinsert 54 to loosen the mold from the die. It is possible that the areamay require a slight machine dressing with an end mill in a drill pressafter molding. The threaded portion of the insert 54 is preferablydesigned with a rounded crown so that the roots of the thread left inthe hub 14 have a sufficient radius to prevent any notch effect orstress risers.

The process of manufacturing the grinding wheel assembly 10 comprisesplacing the abrasive disc 12 in a stationary mold half 32 of the dieassembly 28. Preferably the abrasive disc is preheated to a temperatureapproximately between 150° F. to 200° F. before the thermoplasticinjection step. The movable mold half 30 with the aid of the locatingpins 44 and female mates 50 is moved into position so that the insert 54is in place within the hub cavity of the stationary mold half 32. It isextremely important that the insert 54 be located perpendicular to theplane of the abrasive disc 12 since the abrasive wheel will be utilizedat a high rpm. In one preferred embodiment the die assembly 28 can beheated to a temperature between 150° F. to 200° F. and the abrasive disc12 can likewise either be preheated or brought up to this temperature inthe mold. The cylinder of the molding machine will be heated in therange of 450° F. to 600° F. This temperature range will soften thepolycarbonate. An injection pressure which will be approximately 20,000psi at the orifice of the nozzle can be utilized to push the plunger orram of the injection molding machine forcing the softened polycarbonatethrough the nozzle 36 and the sprue bushing 38 into the cavity thatforms the hub 14. The softened aromatic polycarbonate flows around thethreaded insert 54 that creates the threaded aperture 22 and finallythrough the aperture of the abrasive disc 12 into the circular portion24 on the bottom side of the abrasive disc 12. The pressure of thepolycarbonate around the abrasive disc 12 is approximately 1000 to 1200psi. In a manner of a few seconds the aromatic polycarbonate hashardened enough to permit the timer controlled plunger of the moldingmachine to return to its back position and receive another measuredamount of aromatic polycarbonate into the heating cylinder. After a fewmore seconds the mold will open and the entire grinding wheel assemblywill be stuck to the movable mold half 30 until the ejector pin 51contacts the insert 54 to remove the grinding wheel 10 from the dieassembly 28. The die assembly 28 is then reloaded with another abrasivewheel 12 and another insert 54 with the cycle being repeated. The insert54 is removed from the grinding wheel assembly 10 and any sprue is cutoff. Generally an end mill in a drill press will be required to dressthe finished part and then it would be ready for hot stamping orpainting if desired. The entire operation is performed in approximately30 seconds. It is obvious that a die assembly can consist of a number ofcavities and appropriate insert pins 54 so that several grinding wheelassemblies 10 can be manufactured at the same time. The aromaticpolycarbonate will generally permit the insert 54 to be hand screwed outof the hub 14 while the aromatic polycarbonate is still in a heatedcondition. It is possible to modify the movable mold half 30 to permitan automatic revolving of the insert 54 before or after it has beencontacted by the ejector pin 51 for the purpose of removing the insert54 from the grinding wheel.

It is believed that the use of an aromatic polycarbonate permits notonly a mechanical bonding to the abrasive disc 12 but also permits achemical bond with the thermoset phenolic plastic which is utilized asthe bonding agent in the abrasive disc 12. The combination of both themechanical bond and the chemical bond in a single operation provides ahighly efficient and durable grinding wheel assembly not previouslyavailable to industry.

I claim:
 1. A new material comprised of a thermoplastic resin selectedfrom the group consisting of phenol-containing thermoplastic resins anda cured and molded thermoset phenol-formaldehyde resin bonded to eachother through cross-linking chemical bonds formed after the curing ofsaid thermoset.
 2. The material of claim 1, wherein said thermoplasticresin is a phenol-based polycarbonate.
 3. The material of claim 2wherein said polycarbonate has the following formula: ##STR2## and X isan integer of 50 to
 400. 4. The material of claim 2 wherein thepolycarbonate is an ester of carbonic acid and bis-phenol A.
 5. Thematerial of claim 2, wherein the phenol-formaldehyde resin is aone-stage resin.
 6. The material of claim 2 wherein thephenol-formaldehyde resin is a two-stage resin.
 7. The material of claim6, wherein the molecular weight of said phenol-formaldehyde resin rangesfrom 450 to
 550. 8. The material of claim 1 wherein said cross-linkingchemical bonds are formed by heating said thermoplastic resin to atemperature of about 450° F. to 550° F. and heating said thermoset to atemperature between a 150° F. and 200° F. and then bringing saidthermoplastic and said thermoset into contact with one another.
 9. Thematerial of claim 8 wherein said thermoplastic resin and said thermosetare pressurized against each other after said preheating in a pressurerange between 1000 and 1200 psi to form said chemical cross-linkingbonds.
 10. The chemical area of bonding formed between the thermoplasticresin and the thermoset plastic in the new material of claim 1.